11 Top Javascript Interview Questions (with answers) !

JavaScript is a widely-used programming language that is essential for building interactive and dynamic web pages. As a result, many job interviews for web developer positions include questions about JavaScript. In this article, we will explore 11 common JavaScript interview questions and provide examples and solutions for each.

1. What is the difference between let and var in JavaScript?

The main difference between the ‘let’ and ‘var’ keywords in JavaScript is the way they handle variable scope. Variables declared with ‘var’ have function scope, while variables declared with ‘let’ have block scope. This means that a variable declared with ‘var’ can be accessed within the entire function, while a variable declared with ‘let’ can only be accessed within the block it was declared in.

Example:

function example() { 
var x = 1; 
if (true) { 
  var x = 2; // this will overwrite the x variable declared above 
  console.log(x); // Output: 2 
} 
console.log(x); // Output: 2 
}

function example() { 
  let x = 1;
  if (true) { 
    let x = 2; // this is a different variable, scoped only within the if block 
    console.log(x); // Output: 2 
  } 
  console.log(x); // Output: 1 
}

2. How do you declare a variable in JavaScript?

In JavaScript, variables can be declared using the ‘var’, ‘let’, or ‘const’ keywords. Variables declared with ‘var’ have function scope, while variables declared with ‘let’ and ‘const’ have block scope.

Example:

var x = 1; 
let y = 2; 
const z = 3;

3. What is a closure in JavaScript?

A closure is a function that has access to the variables and functions in the scope where it was created, even after that scope is closed. This allows the function to “remember” its state and continue to access those variables.

Example:

function makeCounter() { 
  let count = 0; 
  return function() { return count++; } 
} 
let counter = makeCounter(); 
console.log(counter()); // Output: 0 
console.log(counter()); // Output: 1

4. What is the difference between == and === in JavaScript?

The ‘==’ operator compares values for equality, while the ‘===’ operator compares both values and types. This means that the ‘===’ operator will only return true if the two values being compared are of the same type and have the same value.

Example:

console.log(1 == '1'); // Output: true 
console.log(1 === '1'); // Output: false

5. How do you declare an array in JavaScript?

Arrays in JavaScript are created using the array literal notation, which is a set of square brackets containing a list of elements separated by commas.

Example:

let myArray = [1, 2, 3];

6. What is hoisting in JavaScript?

Hoisting is the behavior in JavaScript where variable and function declarations are moved to the top of their scope before the code is executed. This means that variables and functions can be used before they are declared in the code.

7. Explain how to create an object in JavaScript

An object in JavaScript can be created using the object literal notation, which is a set of curly braces containing a list of properties separated by commas.

Example:

let myObject = { name: 'John', age: 25, location: 'New York' };

8. Explain how to declare a function in JavaScript.

Functions in JavaScript are declared using the function keyword followed by the function name, a set of parentheses, and a set of curly braces containing the function’s code.

Example:

function myFunction() { console.log("Hello World!"); }

9. Explain the difference between for and forEach loop.

The for loop is used to iterate over a set of statements a specified number of times, while the forEach loop is used to iterate over an array, and execute a function for each element of the array.

Example:

let myArray = [1, 2, 3, 4, 5]; 
// for loop 
for (let i = 0; i < myArray.length; i++) { 
  console.log(myArray[i]); 
} 
// forEach loop 
myArray.forEach(function(element) { console.log(element); });

10. Explain the use of the keyword “this” in JavaScript.

The keyword “this” in JavaScript refers to the object that the function is a property of. It can be used to access and manipulate the properties of the object.

Example:

let myObject = { name: 'John', age: 25, showName: function() { console.log(this.name); } }; 
myObject.showName(); // Output: John

11. Explain the difference between a callback function and a promise in JavaScript.

A callback function is a function that is passed as an argument to another function and is executed after the execution of the parent function. A promise is an object that represents the eventual completion of an asynchronous operation and provides methods for handling success and failure.